1. Field of the Invention
The present invention is directed to stabilizer compositions based on phosphites and siloxane-oxetane compounds, a process for stabilizing polycarbonate and the stabilized polycarbonates produced therefrom.
B 2. Description of the Invention
It is generally known that thermoplastics must be formulated with several additives concurrently in order to improve their properties and their stability. Apart from additives to improve the mold release, the flame resistance and the stability to light and weathering, in particular, stabilizers to improve the resistance to heat, moisture and air are added to the thermoplastics. The additives always contain a particular active group which is responsible for the specific effect of the additives. Accordingly, since it is usually necessary to employ several additives together, and since relatively large amounts of additive have more or less severely unfavorable effects on the physicomechanical properties of the thermoplastics, the aim is obviously to reduce as much as possible the amount added by adding those additives which have the lowest possible equivalent weight relative to their particular active group. On the other hand, this aim is restricted by the fact that the additives should have as low a volatility as possible and thus have as high molecular weight as possible, so that they cannot diffuse out of the thermoplastic again during or after its processing.
Thus, an object of the present invention is to provide stabilizers against thermo-oxidative degradation of polycarbonate which comply with these general requirements, that is to say additives which combine as low an active group equivalent weight as possible with as low a volatility as possible.
A further object is not to adversely affect the stability of the polycarbonate to hydrolysis by the addition of the stabilizers.
The stabilization of polycarbonate by the addition of phosphites has been known for a considerable time. However, the action of the phosphites principally involves reducing highly color-producing degradation products, which are formed during the preparation of the polycarbonate or its processing, to give colorless products (Levantowskaja et al, vysokom, Soed. X (1975), pages 2331-37).
The disadvantage of stabilization with phosphites is the severe brown discoloration of the substrate on relatively long exposure to air and heat. The long-term stability can be improved by the addition of specific phosphites having oxetane groups or by the addition of mixtures of phosphites and oxetane compounds (see German Offenlegungsschrift No. 2,140,207, German Offenlegungsschrift No. 2,510,463 and German Offenlegungsschrift No. 2,658,849).
It has now been found that a particularly favorable stabilizing action is obtained when combinations of phosphites and specific siloxanes containing oxetane groups are used. This is because these oxetane groups have, on the one hand, a relatively low equivalent weight per oxetane group and, on the other hand, nevertheless have a relatively low volatility.
This distinguishes the silicon-oxetane compounds of the present invention from the C-oxetane compounds described in German Offenlegungsschrift No. 2,510,463. It is true that silicon compounds are already mentioned in the form of esters of silicic acid with oxetane-containing alcohols in this patent specification. However, as is shown in the examples of the present application, the resistance to hydrolysis achieved with those type of silicic ester/phosphite mixtures is poorer than when using the phosphite/silicon-oxetane compositions according to the present invention. When using these stabilizer compositions, in some cases, the resistance to hydrolysis of unstabilized polycarbonate can even be exceeded. Moreover, the silicic esters of the formula ##STR1## mentioned in German Offenlegungsschrift No. 2,510,463 are, in the case where R=CH.sub.3 or C.sub.2 H.sub.5, solid crystalline compounds. Obviously, metering compounds of this type into the polycarbonate, especially during a continuous process of manufacture, is more difficult and demands additional chemical engineering measures, in contrast to the use of the siloxanes according to the invention, which are liquid at room temperature.
Although, on the one hand, it is known that silicon-oxetane compounds do not adversely affect the stability to hydrolysis in phosphite-free polycarbonate (see German Offenlegungsschrift No. 2,920,450 and German Offenlegungsschrift No. 2,902,451) and although, on the other hand, it is known that the stability to hydrolysis of phosphite-stabilized polycarbonates can be improved by mixing in carboxylic esters containing oxetane groups (see German Offenlegungsschrift No. 2,658,849), it was nevertheless surprising that excellent results could be achieved with the inventive compositions with respect to both of stabilization towards thermal oxidation and stability to hydrolysis. This is surprising because it can be seen from German Offenlegungsschrift No. 2,726,662 that the stability to hydrolysis of phosphite-stabilized polycarbonate is adversely affected by the addition of the siloxane compounds mentioned therein (Example 3 of German Offenlegungsschrift No. 2,726,662).
Regarding phosphite-free siloxane stabilization according to German Offenlegungsschrift No. 2,920,450 and German Offenlegungsschrift No. 2,920,451, it has been found that the discoloration-reducing effect of this stabilization is very dependent on the batch and, in particular, is ineffective against certain color-producing degradation products which can only be decolorized by reducing agents.
Since in the course of continuous large-scale production certain variations in quality within a specified range of tolerance must always be expected, stabilization with siloxane but without phosphite leads to variable and thus unsatisfactory results with respect to the lightness of the color of the polycarbonate. In contrast, when using the stabilizer compositions according to the invention, completely uniform color quality can be guaranteed, irrespective of the batch.